Electrode

ABSTRACT

An electrode has a current collector having a plurality of through holes, and active material layers provided on both sides of the current collector, the current collector has projections extending on the top side or on the back side of the current collector from respective edges of the through holes, and an angle between each of the projections and a surface direction of the current collector is in the range of 30 to 80°. Because of the configuration wherein the projections are surrounded by the active material layers, a physical anchor effect is achieved so as to provide high adhesion. Since the projections are inclined relative to the surface direction of the current collector, the distance becomes shorter between the current collector including the projections and an active material, which was located apart from the projections in the case where the projections extended perpendicularly to the surface direction of the current collector. Therefore, electrically conducting paths become shorter and impedance is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode.

2. Related Background Art

Electrochemical devices such as primary batteries, secondary batteries(particularly, lithium-ion secondary batteries), electrolysis cells, andcapacitors (particularly, electrochemical capacitors) are widely used invarious fields. An electrode for such electrochemical devices isconstructed normally in such a structure that an active material layeris laid on a plate-like current collector having through holes and thisactive material layer contains a large number of particles containing anactive material.

It is important for the foregoing electrode to improve adhesion betweenthe current collector and the active material layer, in order to achieveimprovement in performance. For this reason, a variety of techniqueshave been researched in order to improve the adhesion between thecurrent collector and the active material, layer. For example, JapanesePatent Application Laid-open No. 2004-103462 (Patent Literature 1) showsa core member (current collector) for an electrode with a plurality ofholes made by piercing a metal foil member from both of the top and backsides and leaving curves or burrs extending perpendicularly to the bothsides.

SUMMARY OF THE INVENTION

The technology described in Patent Literature 1 improves the adhesionbetween the current collector and the active material layer because ofthe existence of the curves or burrs on the surface of the currentcollector, but has a problem that there is variation in electricallyconducting paths between the active material in the active materiallayer and the current collector. Namely, the distance between thecurrent collector and the active material is short near a curve or burrand thus the electrically conducting paths are short there. However, theactive material located away from the curves or burrs is distant fromthe current collector and the electrically conducting paths becomelonger. As a result, the electrode failed to achieve sufficientreduction in impedance and it was a hindrance to further improvement inoutput characteristics.

The present invention has been accomplished in light of theabove-described circumstances and an object of the invention is toprovide an electrode with high adhesion between the current collectorand the active material layer and with low impedance.

In order to achieve the above object, an electrode according to thepresent invention is an electrode comprising a current collector havinga plurality of through holes, and an active material layer provided on asurface of the current collector, wherein the current collector hasprojections extending from edges of the through holes to the outside ofthe through holes, and an angle between each of the projections and thesurface of the current collector is in the range of 30 to 80°.

According to the present invention, the angle between each of theprojections extending from the current collector of the electrode andthe surface of the current collector is in the range of 30 to 80° andthe inclined projections are surrounded by the active material layer;therefore, a physical anchor effect is achieved, so as to realize highadhesion between the current collector and the active material layer.Since the projections are inclined relative to the surface of thecurrent collector, the distance becomes shorter between the currentcollector including the projections, and the active material, which waslocated apart from the projections extending perpendicularly to thesurface of the current collector in the conventional technology.Therefore, the electrically conducting paths become shorter and theimpedance is reduced.

When the angle between each projection and the surface of the currentcollector is in the range of 40 to 60°, the impedance is more suitablyreduced.

A ratio of a distance between a tip of the projection and the surface ofthe current collector to a thickness of the active material layer ispreferably in the range of 0.24 to 0.99. In this case, it is feasible tosuitably maintain the high adhesion between the current collectorincluding the projections, and the active material layer and to moreeffectively reduce the impedance.

A configuration for effectively achieving the above-described action isspecifically a configuration wherein the through holes are quadrangular,wherein the projections are provided on respective sides forming theedges of the through holes so as to extend each to the outside of thethrough holes, and wherein a pair of projections extending from twoadjacent sides of each through hole, out of the projections provided onthe respective sides, extend in a direction to close the through holeand the other pair of projections extending from the sides differentfrom the two adjacent sides extend in a direction to extend away fromthe through hole.

The present invention provides the electrode with high adhesion betweenthe current collector and the active material layer and with lowimpedance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram showing an electrode 1according to a preferred embodiment of the present invention.

FIG. 2 is a sectional view of the electrode 1 according to the preferredembodiment of the present invention.

FIG. 3 is a schematic configuration diagram showing an electrodeproducing apparatus 100 for producing the electrode 1 according to thepreferred embodiment of the present invention.

FIG. 4 is a drawing to illustrate a current collector sheet 2A flowingin the electrode producing apparatus 100.

FIG. 5 is a sectional view showing a configuration of a conventionalelectrode 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the present invention will be describedbelow in detail with reference to the accompanying drawings. In thedescription of the drawings identical or similar elements will bedenoted by the same reference symbols, without redundant description.

FIG. 1 is a schematic configuration diagram showing an electrode 1according to the preferred embodiment of the present invention. FIG. 2is a sectional view of the electrode 1.

The electrode 1 of the present embodiment, as shown in FIG. 1, iscomposed of a current collector 2, and active material layers 3 disposedon the top side and on the back side, respectively, of the currentcollector 2. The electrode 1 shown in FIG. 1 is suitably applicable toelectrochemical devices such as primary batteries, secondary batteries(particularly, lithium-ion secondary batteries), electrolysis cells, andcapacitors (particularly, electrochemical capacitors).

There are no particular restrictions on the current collector 2 as longas it is an electrically conductive sheet; for example, preferablyapplicable materials are copper, aluminum, and so on. There are noparticular restrictions on the thickness and shape of the currentcollector 2, but the current collector 2 can be, for example, abelt-like sheet having the thickness (corresponding to thickness T inFIG. 2) in the range of 10 to 30 μm and the width in the range of 50 mmto 2000 mm.

The active material layers 3 are formed on the top side and on the backside, respectively, of the current collector 2. The active materiallayers 3 are layers containing a positive-electrode ornegative-electrode active material. The active material can be any oneof known materials. For example, positive-electrode active materials forlithium secondary batteries are, typically, lithium oxides such asLiCoO₂ and LiMn₂O₄, and another applicable active material is one or acombination of two or more species of chalcogen compounds such as TiS₂,MnO₂, MoO₂, and V₂O₅. Negative-electrode active materials for lithiumsecondary batteries preferably used are lithium, lithium alloys, orcarbonaceous materials such as graphite, carbon black, and acetyleneblack. Examples of electrodes for electric double layer capacitorsinclude a variety of porous materials with electron conductivity. Forexample, preferably applicable materials include carbon materials suchas natural graphite, artificial graphite, mesocarbon microbeads,mesocarbon fiber (MCF), cokes, glasslike carbon, and burned substancesof organic compounds.

The active material layers 3 further contain, a binder and others, inaddition to the aforementioned active material. It may contain anelectrical conducting agent or the like if necessary. There are noparticular restrictions on the binder forming the active material layers3, as long as it can secure the foregoing active material, electricalconducting agent, etc. to the current collector; it can be one ofvarious binding agents. For example, applicable binders includefluorocarbon polymers such as polyvinylidene fluoride (PVDF) andpolytetrafluoroethylene (PTFE), mixtures of styrene-butadiene rubber(SBR) and water-soluble polymers (carboxymethyl cellulose, polyvinylalcohol, sodium polyacrylate, dextrin, gluten, etc.), and so on. Theelectrical conducting agent can be one selected, for example, fromcarbon blacks, metal fine powders of copper, nickel, stainless steel,iron, etc., mixtures of carbon material and metal fine powder, andelectroconductive oxides such as ITO (Indium Tin Oxide).

The active material layers 3 are formed by Mixing the above-describedmaterials, for example, with a solvent such as ether or ketone, kneadingor dispersing them to prepare a coating solution for formation of activematerial layer, applying the coating solution onto the current collector2, and drying it. The details thereof will be described later.

The current collector 2 is provided with through holes 4 and hasprojections 4 a extending on the top side of the current collector 2 andprojections 4 b extending on the back side of the current collector 2from the edges of the through holes 4. There are no particularrestrictions on a method for forming the through holes 4 and projections4 a, 4 b, but the projections (4 a, 4 b) are formed, for example, bypiercing an electroconductive sheet used as the current collector 2, tobreak the electroconductive sheet at hole portions (to become thethrough holes 4) with a tool or the like from both sides, and thereby tobend the sheet in a piercing direction of the tool. The method offorming the through holes 4 and projections 4 a, 4 b can beappropriately modified, for example, by altering the shape at the tip ofthe tool so as to facilitate formation of projections 4 a, 4 b on theoccasion of forming the through holes 4. The current collector 2 in thepresent embodiment is provided with the rhombic through holes 4, but theshape of the through holes 4 may be a quadrangular shape or any othershape such as a circle. The shape of the through holes 4 can be changedby the shape of the tool or the like for formation of the through holes4, the method of forming the through holes 4, and so on. For example,when the tip of the tool is of a quadrilateral pyramid shape, thethrough holes 4 are readily formed in a quadrilateral shape as shown inFIGS. 1 and 4. The current collector preferably applicable is, forexample, a pricked metal sheet obtained by forming the through holes 4in the electroconductive sheet with the tool of the shape of throughholes 4 as described above, or an expanded metal sheet obtained byforming cuts in a zigzag pattern in the electroconductive sheet andexpanding the cuts into a rhombic or hexagon shape.

As shown in FIG. 2, the projections 4 a, 4 b extend on the top side andon the back side, respectively, of the current collector 2. When theprojections 4 a, 4 b are made by an ordinary forming method, they areformed on the occasion of forming the through holes 4 and thus the sizethereof varies depending upon the size of through holes 4. The size ofprojections 4 a, 4 b also varies depending upon a situation of breakageof the electroconductive sheet with the tool, but the length a from theedge of through hole 4 is preferably in the range of 30 μm to 100 μm andparticularly preferably 70 μm. When the length a of projections 4 a, 4 bfalls within this range, the projections 4 a, 4 b are widely arranged inthe active material layers 3. When the length a of projections 4 a, 4 bis in the foregoing range, the thickness L of each of the activematerial layers 3 formed on the top side and on the back side of thecurrent collector 2 is preferably in the range of 50 to 200 μm.Furthermore, when H represents a distance between the tip of eachprojection 4 a, 4 b and a surface of the sheet part (main body part) ofthe current collector 2, a ratio (H/L) of distance H to thickness L ispreferably in the range of 0.24 to 0.99. When the ratio H/L is in theforegoing range, the provision of projections 4 a, 4 b offers greatershortening of electrically conducting paths and reduction in impedanceresulting therefrom.

An angle between each projection 4 a, 4 b and the sheet part (main bodypart) of the current collector 2 is preferably in the range of 30 to 80°and more preferably in the range of 40 to 60°. The angle between theprojection 4 a, 4 b and the sheet part of the current collector 2 hereinrefers to, for example, angle A1 shown in FIG. 2. Specifically, theangle A1 is an acute angle part out of angles made between the surfacepart of the sheet and a tangent line to the tip part of projection 4 a,at an intersection of an extension of the tangent line to the tip partof projection 4 a with the surface part of the sheet of the currentcollector 2. When the projections 4 a, 4 b have an inclination in theforegoing range, instead of extending vertically to the currentcollector 2, as described above, a physical anchor effect is achieved,so as to provide an effect of enhancing adhesion between the currentcollector 2 including the projections 4 a, 4 b, and the active materiallayers 3. When the projections 4 a, 4 b have the inclination in theaforementioned range, the electrode further has an effect of shorteningelectrically conducting paths between the active material in the activematerial layers 3 around the projections 4 a, 4 b and the currentcollector including the projections 4 a, 4 b.

In the current collector 2 shown in FIGS. 1 and 2, the rhombic throughholes 4 are formed and the projections 4 a, 4 b extend from therespective sides of the rhombic shape forming the edges of through holes4. Some of four projections 4 a, 4 b extending from one through hole 4are inclined in a direction to cover the through hole 4, while the otherprojections 4 a, 4 b are inclined in a direction to extend away from thethrough hole 4. As a consequence, all the projections 4 a, 4 b on thecurrent collector 2 are configured so as to be inclined in the samedirection (to the left in FIG. 2) and the angles between the projections4 a, 4 b and the main body part of the current collector 2 fall withinthe aforementioned range. In the current collector 2 forming theelectrode 1 in the present embodiment, all the projections 4 a, 4 b areinclined in the same direction as described above, but they do notalways have to be inclined in the same direction. Namely, it issufficient that the angles of the projections 4 a, 4 b with the sheetpart of the current collector 2 fall within the aforementioned range,and there are no particular restrictions on the direction of inclinationthereof. Even if the projections 4 a, 4 b are inclined in differentinclination directions, the inclination of the projections 4 a, 4 b alsopresents the foregoing effects of improvement in adhesion and reductionin impedance.

There are no particular restrictions on the shape and size of throughholes 4 formed in the current collector 2, either, but the through holes4 are preferably formed in such size that the length a of projections 4a, 4 b falls within the aforementioned range (30 μm to 100 μm); forexample, the through holes 4 can be formed in the rhombic shape witheach side of 150 μm as shown in FIG. 1. There are no particularrestrictions on the number of through holes 4, either, but the throughholes 4 with the projections 4 a, 4 b are preferably provided in thenumber not to significantly degrade the durability of the currentcollector 2, while enhancing the aforementioned effect of shorteningelectrically conducting paths.

The effect of shortening electrically conducting paths will be describedbelow using FIGS. 2 and 5. FIG. 5 is a sectional view showing aconfiguration of a conventional electrode 5. In the conventionalelectrode 5, projections 4 a, 4 b extend each perpendicularly to thecurrent collector 2 from the edges of through holes 4 of the currentcollector 2. Therefore, the active material in the active materiallayers 3 near the projections 4 a, 4 b is located near the currentcollector 2 and thus the electrically conducting path is short betweenthem. However, as to a point, for example, like point Y in FIG. 5, whichis located above a through hole 4 and at which the projection 4 bextends in the opposite direction to the point Y (on the back side ofthe current collector), the point becomes farther from the projections 4a, 4 b and the main body of the current collector 2 and thus theelectrically conducting path becomes longer between them. Therefore,this longer electrically conducting path increases the impedance and itwas infeasible to achieve sufficient reduction in impedance as a wholeof electrode 5.

On the other hand, in the electrode 1 of the present embodiment, wherepoint X is defined at a point which is located above a through hole 4like point Y in FIG. 5, and at which the projection 4 b extends in theopposite direction (on the back side of the current collector) to thepoint Y, the distance from the main body (sheet part) of the currentcollector 2 is the same as in the electrode 5 in FIG. 5, but thedistance between the projection 4 a and the point X becomes shorter,when compared with the point Y in FIG. 5, because the projection 4 aextending on the top side of the current collector 2 is inclined towardthe point X. Therefore, the electrically conducting path by the activematerial at point X becomes shorter than the electrically conductingpath by the active material at point Y. Since the electricallyconducting paths of the active material can be shortened when comparedwith the conventional electrode 5 as described above, the impedance canbe adequately reduced as a whole of the electrode 1 and thus outputcharacteristics can be improved thereby. In addition, the projections 4a, 4 b adhere tightly to the active material layers 3, whereby highadhesion can also be maintained between the current collector 2 and theactive material layers 3.

The below will describe a method of producing the above electrode 1,using FIGS. 3 and 4.

As shown in FIG. 3, an electrode producing apparatus 100 for producingthe electrode 1 according to the present embodiment is constructed witha coating solution tank 10, a coating solution supply pump 11, a slitdie 12, a sheet supply reel 20, a backup roll 21, a nip roll 22, a drier25, and a take-up reel 30.

The coating solution tank 10 stores a coating solution for formation ofactive material layer. The coating solution for formation of activematerial layer contains the aforementioned active material, binder,electrical conducting agent, solvent, and so on. The viscosity of thecoating solution is, for example, preferably in the range of 50 P to 500P and more preferably in the range of 100 to 300 P

A supply line L1 connects between the coating solution tank 10 and theslit die 12. The pump 11 which supplies the solution in the coatingsolution tank 10 to the slit die 12 at a constant rate is connected tothe supply line L1. The pump 11 to be used herein can be, for example, aprecision gear pump.

The sheet supply reel 20 is a reel which supplies the current collectorsheet 2A, onto which the coating solution for formation of activematerial layer is to be applied, to the electrode producing apparatus100. The take-up reel 30 is a reel which takes up the current collectorsheet 2A having passed through the steps of being supplied from thesheet supply reel 20, being coated with the coating solution forformation of active material layer, and being dried by the drier 25 toform the active material layer. An unrepresented motor is connected tothe take-up reel 30 and has a function to take up the current collectorsheet 2A through rotation of the take-up reel 30 at a fixed rate.

The projections 4 a, 4 b are preliminarily formed on both sides of thecurrent collector sheet 2A used in the electrode producing apparatus 100of the present embodiment. There are no particular restrictions on howto form the projections 4 a, 4 b, as described above.

The backup roll 21 is a rotatable roll of a cylindrical shape. Thecurrent collector sheet 2A which is supplied from the sheet supply reel20 and which is taken up by the take-up reel 30 is wound around thebackup roll 21 whereby the current collector sheet 2A is guided. Thereare no particular restrictions on the diameter of the backup roll 21,but the outside dimension thereof can be, for example, in the range of10 to 250 mm. There are no particular restrictions on a rotating speedof the backup roll 21, but it is preferable to set a linear velocity onthe peripheral surface of the backup roll 21 so as to be equal to a linespeed (take-up speed of the current collector sheet 2A by the take-upreel 30), for the purpose of preventing a sag of the current collectorsheet 2A wound.

The slit die 12 has a slit 12 b provided with an opening along the axialdirection of the backup roll 21. This slit die 12 is configured tospread the liquid flowing thereinto from an entrance of the slit die 12,in the width direction of the current collector sheet 2A in a manifold12 a provided as a hollow space inside the slit die 12 and extending inthe axial direction of the backup roll 21, and to discharge the liquidin a sheet form through the slit 12 b and from the exit opening. Thisstep is to apply the coating solution 3A onto the surface of the currentcollector sheet 2A. The thickness of a coating solution layer formed onthe surface of the current collector sheet 2A is preferably in the rangeof 50 to 200 μm.

The coating solution 3A applied on the surface of the current collectorsheet 2A is dried by the drier 25 provided on the way of movement by thetake-up reel 30. The drier 25 can be a hot-wire heater, a steam heater,an infrared heater, or the like.

The nip roll 22 is disposed between the sheet supply reel 20 and thebackup roll 21 and is arranged so that the axis of the nip roll 22becomes parallel to the axis of the backup roll 21. This nip roll 22 hasa function to wind the current collector sheet 2A supplied from thesheet supply reel 20, around the backup roll 21. Furthermore, the niproll 22 has a function to incline the projections 4 a, 4 b on thecurrent collector sheet 2A by pressing the collector sheet 2A againstthe peripheral surface of the backup roll 21 on the occasion of windingthe current collector sheet 2A around the backup roll 21.

The below will detail a method of inclining the projections 4 a, 4 b onthe current collector sheet 2A by press by the nip roll.

When the nip roll 22 is arranged so that a distance W1 between theperipheral surface of the nip roll 22 and the peripheral surface of thebackup roll 21 is smaller than the sum of the thickness of the currentcollector sheet 2A and the length of the projections 4 a, 4 b formedperpendicularly to the current collector sheet 2A (i.e., the totalthickness including the main body part of the current collector and theprojections 4 a, 4 b), the sheet is moved along the peripheral surfacesof the nip roll 22 and backup roll 21 while the projections 4 a, 4 b arepressed each against the current collector sheet 2A, whereby theprojections 4 a, 4 b are inclined in the direction opposite to theflowing direction of the current collector sheet 2A. The distance W1 isnormally made approximately equal to the thickness of the currentcollector sheet 2A.

FIG. 4 is a drawing to illustrate the current collector sheet 2A flowingin the electrode producing apparatus 100. In FIGS. 4, (A) and (B) areviews showing the current collector sheet 2A immediately after theprojections 4 a, 4 b are inclined by the nip roll 22 and the backup roll21, wherein FIG. 4 (A) is a view from the top side of the currentcollector sheet 2A traveling in a direction indicated by arrow A (to theright in FIG. 4) and FIG. 4 (B) is a sectional view along line IVB-IVBthereof. As shown in (A) and (B) of FIG. 4, the projections 4 a, 4 bextending from the through holes 4 all are inclined in the directionopposite to the moving direction. In the case where the through holes 4provided in the current collector sheet 2A are rhombic and where adiagonal line thereof is parallel to the moving direction (arrow A) ofthe current collector sheet 2A, as shown in FIG. 4 (A), the projections4 a, 4 b provided on the edges on two adjacent sides out of the foursides forming the rhombic shape (in the case of FIG. 4 (A), the twosides on the right side being the fore side in the moving direction withrespect to each through hole 4) are inclined in a direction to close thethrough hole 4. On the other hand, the projections 4 a, 4 b provided onthe edges on the other two sides (the two sides on the left side beingthe back side in the moving direction with respect to each through hole4) are inclined in a direction to extend away from the through hole 4.When all the sides forming the through holes 4 are present in directionsdifferent from the moving direction of the current collector sheet 2A asdescribed above, the projections 4 a, 4 b formed on the sides becomeinclined without being crushed by pressing under movement in theelectrode producing apparatus 100.

The linear pressure in pressing the current collector sheet 2A ispreferably in the range of 2 to 50 kgf/cm and more preferably in therange of 5 to 15 kgf/cm though it depends on the material and thicknessof the current collector sheet 2A, the length of the projections 4 a, 4b, and so on. If the linear pressure is larger than 50 kgf/cm, thecurrent collector sheet 2A can be damaged as the projections 4 a, 4 bare inclined. If the linear pressure is smaller than 2 kgf/cm, theprojections 4 a, 4 b can fail to be adequately inclined.

In the electrode producing apparatus 100 shown in FIG. 3, the nip roll22 presses the sheet against the peripheral surface of the backup roll21 to incline the projections 4 a, 4 b of the current collector sheet 2Ain the direction opposite to the moving direction, and then the currentcollector sheet 2A is continuously conveyed along the backup roll 21 tothe opening of the slit die 12. The coating solution supplied from theslit die 12 is delivered onto the surface of the current collector sheet2A (the surface on which the projections 4 a are formed). FIG. 4 (C) isa sectional view of the current collector sheet 2A under supply of thecoating solution 3A in an inclined state of the projections 4 a, 4 b. Inthis manner, the coating solution 3A is applied to the surroundings ofthe inclined projections 4 a. Then the coating solution is dried to formthe active material layer on one side of the current collector sheet 2A.

If the projections 4 a on the current collector sheet 2A catch on theslit die 12, there can arise such problems as failure in application dueto occurrence of positional deviation of the current collector sheet 2Aand breakage of the current collector sheet 2A, as well as a problem ofuneven thickness of coating film. In the case of the current collectorsheet 2A of the present embodiment, however, the projections 4 a areinclined in the direction opposite to the conveyance direction of thecurrent collector sheet 2A and thus inclined in the same direction asthe moving direction of the slit die 12 relative to the currentcollector sheet 2A. Therefore, this configuration suppresses occurrenceof such an accident that the projections 4 a face the slit die 12 andcatch thereon at the tip; as a result, it can reduce occurrence of thevarious problems to be expected in the event that the projections 4 acatch on the slit die 12.

After this process, the current collector sheet 2A with the activematerial layer on one side is arranged so that the other side becomesthe surface to be coated with the coating solution 3A, in the currentcollector sheet 2A (in the present embodiment, so that the back side ofthe current collector sheet 2A is the surface to be coated). Then thenip roll 22 again presses the current collector sheet 2A to incline theprojections and then the coating solution 3A is continuously applied anddried. The above results in forming the active material layers on bothsides of the current collector sheet 2A. Thereafter, this sheet is cutin a predetermined size to obtain the electrode 1 according to thepresent embodiment.

In the electrode producing method using the electrode producingapparatus 100 according to the present embodiment, as described above,the active material layers are formed in such a manner that the coatingsolution 3A is applied onto the current collector sheet 2A in a state inwhich the projections 4 a, 4 b on the current collector sheet 2A areinclined by press by the nip roll 22. Therefore, the projections areinclined in the definite direction as shown in FIG. 2, so that theelectrode 1 can be produced with reduced impedance. At the same time,since the coating solution is supplied at a point of time in a thermallyactive state, the adhesion between the current collector and the activematerial layers is further enhanced when compared with the conventionalcase using punched metal.

In the case of the electrode producing method using the electrodeproducing apparatus 100, when the projections are formed so as tooriginate from the sides extending in the directions different from themoving direction of the current collector sheet 2A moving in theelectrode producing apparatus 100 as shown in FIG. 4 (A), it is feasibleto reduce breakage of projections due to the press on the projections,and thereby to more effectively realize reduction of impedance andimprovement in adhesion.

The above described the preferred embodiment of the present invention,but it should be noted that the present invention is by no means limitedto the above embodiment but can be modified in various ways.

For example, the electrode producing apparatus 100 according to theembodiment of the present invention was described using the slit die 12as a device for applying the coating solution, but any other device canbe used as long as it is a device capable of continuously applying thecoating solution after the inclining step of the projections 4 a, 4 b.For example, it is possible to adopt a configuration in which thecoating solution is applied by use of a mill roller, a doctor blade, orthe like.

The electrode producing apparatus 100 of the present embodiment wasdescribed as to the method of inclining the projections 4 a, 4 b bypressing the current collector sheet 2A by the nip roll 22, but it isnoted that the projections 4 a, 4 b can also be inclined by anothermethod. For example, it is also possible to adopt a method of incliningthe projections 4 a, 4 b on the current collector sheet 2A by lettingthe current collector sheet pass through a slit having a space not lessthan the thickness of the main body part of the current collector sheet2A excluding the projections 4 a, 4 b and smaller than the totalthickness of the current collector sheet 2A including the projections 4a, 4 b.

The current collector sheet 2 of the electrode 1 according to theembodiment of the present invention is provided with the inclinedprojections 4 a, 4 b on both of the top and back sides, but it is alsopossible to adopt a configuration wherein the projections are formed ononly one side. This configuration also offers the effects of achievinghigh adhesion between the inclined projections and the active materiallayer 3 formed on the surface on which the projections are provided, andreduction in impedance.

The present embodiment was described as to the method of forming thevertically extending projections on the occasion of preliminarilyforming the through holes in the current collector sheet 2A, incliningthe projections by the electrode producing apparatus 100, and thereafterapplying the coating solution to form the active material layer, but theelectrode according to the present embodiment can also be produced byanother method different from the foregoing method. For example, it isalso possible to adopt such a method that in a case where the throughholes are formed by piercing the current collector sheet 2A with a tool,the tip of the tool is shaped with an inclination relative to the edgesand the piercing length through the current collector sheet with thetool is adjusted so as to provide the current collector sheet with theprojections having the same inclination as the shape of the tip of thetool.

Example 1

A copper current collector sheet 20 μm thick was perforated to formrhombic through holes with the length of projections (burrs) being 70 μmin such arrangement that the rhombic through holes were alternatelyprojecting at equal intervals of 500 μm from the top and back sides. Theprojections of this current collector sheet were inclined using theaforementioned electrode producing apparatus 100 and then the coatingsolution was applied onto the collector sheet to form the activematerial layer, thereby producing an electrode. At this time, thediameter of the nip roll 22 in the electrode producing apparatus 100 was120 mm, the diameter of the backup roll 21 was 120 mm, and the linespeed was 8 mm/min. The coating solution for formation of the activematerial layer was a solution obtained by mixing and dispersing 90 partsby mass of black lead (trade name: OMAC available from Osaka Gas Co.,Ltd.) and 1 part by mass of graphite (trade name: KS-6 available fromLONZA) as an active material, 2 parts by mass of carbon black (tradename: DAB available from DENKI KAGAKU KOGYO KABUSHIKI KAISHA) as anelectrical conducting assistant, and 7 parts by mass of polyvinylidenefluoride (trade name: KYNAR 761 available from ATFINA) as a bindingagent, thereafter adding an appropriate amount of N-methyl-pyrrolidone(NMP) as a solvent to adjust viscosity, and thereby preparing a slurrycoating solution for negative electrode. The viscosity of the coatingsolution was 270 P. When the current collector sheet was pressed underthe linear pressure of 40 kgf/cm by the nip roll, the angle between eachprojection and the current collector sheet 2 after completion of theinclining process by the nip roll 22 was 30°. Then the coating solutionwas applied so that the thickness L of the active material layer was 150μm. The current collector sheet coated with the coating solution wasdried to form the active material layer on one side. At this time, theratio H/L of the distance H between the tip of the inclined projectionsand the surface of the current collector sheet to the thickness L of theactive material layer was 0.23. The active material layer was formed onthe other side by the electrode producing apparatus 100, and thereafterthe current collector sheet with the active material layers on bothsides was cut to obtain an electrode of Example 1. The impedance of thiselectrode was measured.

Examples 2-12 and Comparative Examples 1-4

Electrodes of Examples 2-12 and Comparative Examples 1-4 were preparedby the same method in Example 1, except that the linear pressure inpress by the nip roll 22 and the thickness L of the active materiallayer were changed to the conditions shown in Table 1. Among them,Comparative Example 1 was prepared using a current collector sheetobtained by forming the through holes and then removing the projectionstherefrom. As a consequence, the angle between the projections and thecurrent collector sheet and H/L of each electrode were the valuespresented in Table 1. The impedance was measured for each of theseelectrodes.

Table 1 shows the results of Examples 1-12 and Comparative Examples 1-4.As shown in Table 1, it was confirmed that the impedance was reduced inExamples 1-12 as compared with Comparative Examples 1-4. The electrodeof Example 3 had the structure in which the tip of some projections wasprojecting out from the active material layer, and a short circuitoccurred in use as an electrode in some occasions.

TABLE 1 Thickness Linear L of pressure active Burr An- in materiallength gle processing layer Impedance [μm] [°] [kgf/cm] [μm] H/L [mΩ]Comparative — 0 — 150 — 12.5 Example 1 Comparative 70 25 50 150 0.2011.9 Example 2 Comparative 70 85 1.5 150 0.46 11.4 Example 3 Comparative70 90 0 150 0.47 10.9 Example 4 Example 1 70 30 40 150 0.23 9.4 Example2 70 40 20 150 0.30 7.1 Example 3 70 45 15 45 1.10 6.7 Example 4 70 4515 50 0.99 5.7 Example 5 70 45 15 100 0.49 6.1 Example 6 70 45 15 1500.33 6.2 Example 7 70 45 15 200 0.25 6.4 Example 8 70 45 15 210 0.24 7.8Example 9 70 50 10 150 0.36 6.8 Example 10 70 60 5 150 0.40 7.1 Example11 70 70 3 150 0.44 9.3 Example 12 70 80 2 150 0.46 9.6

1. An electrode comprising a current collector having a plurality ofthrough holes, and an active material layer provided on a surface of thecurrent collector, wherein the current collector has projectionsextending from edges of the through holes to the outside of the throughholes, and an angle between each of the projections and the surface ofthe current collector is in the range of 30 to 80°.
 2. The electrodeaccording to claim 1, wherein the angle between each of the projectionsand the surface of the current collector is in the range of 40 to 60°.3. The electrode according to claim 1, wherein a ratio of a distancebetween a tip of the projection and the surface of the current collectorto a thickness of the active material layer is in the range of 0.24 to0.99.
 4. The electrode according to claims 1, wherein the through holesare quadrangular, wherein the projections are provided on respectivesides forming the edges of the through holes so as to extend each to theoutside of the through holes, and wherein a pair of projectionsextending from two adjacent sides, out of the projections provided onthe respective sides of each through hole, extend in a direction toclose the through hole and the other pair of projections extending fromthe sides different from the two adjacent sides extend in a direction toextend away from the through hole.